Low power lasers are employed to read and write binary data on the data side of media. One typical media are optical storage discs, such as CDs, DVDs and the like. Typically, various types of data are written on the data side of the disc by a laser beam while the disc is rotating. Data may be recorded by changing a property of a desired area on the recording media so that the area is indicative of a zero or one data value.
The side of a data disc opposite the data side is often used for handwriting or affixing or marking a label with descriptions and illustrations corresponding to the recorded data. Recently, apparatus and methods have been developed with the ability to generate an optically visible label on the non-data region of an optical disc using the same laser that was employed to read and write digital or electronic data on the data side of the disc. See U.S. Patent Application Publication No. 2003/0108708 (Anderson, et al.), disclosing the use of laser sensitive materials on a disc label that react chemically with the application of light and heat and result in changes in color and shading on the label.
In making marks on a disc using an optical disc drive, the laser power delivered to the media may be reduced over time through, for example, attrition of laser power or contamination build-up on optics associated with the laser. This reduction in laser power may adversely affect the ability of the laser to reliably read or write to the disc or other media. Consequently, the calibration of laser power to front sense diode voltage that was set in the factory may vary significantly during usage. Accordingly, an adjustment of the laser power may be needed to maintain good optical density on the marks being formed on the label areas and reliable recording and detecting of binary data on the data areas.
One approach to adjusting laser power has utilized direct reading of the data recording media during writing (DRDW), in which a detection system monitors the reflection of the write signal during the write operation and measures it against a predetermined standard. Another method uses a method of direct read after writing (DRAW), in which a dual laser beam array provides two overlapping beams of the same wavelength, each of which are monitored by separate detectors. One beam is used for data writing and the other reads the data directly after writing. The power levels of the two overlapping reflected beams are compared to monitor and adjust the power output. In marking on the label side of the disc, using the technology described in the Anderson application, the chemistry on the label is optimized to absorb as much light as possible at the wavelength of the writing laser beam. As a result, there is very little reflectivity of light at the wavelength of the writing laser, and the change in reflectivity between written and unwritten areas is very small. Accordingly, the use of a writing laser to also generate light to measure the reflectivity of the written mark is not likely to yield satisfactory results.